Strong Acids and Bases

Skills to develop

Strong Acids and Bases

The animation here shows the formation of H3+O ions
and OH- ions in an aqueous system.

Acids and bases that are completely ionized when dissolved in water
are called strong acids and strong bases
There are only a few strong acids and bases, and everyone should
know their names and properties. These acids are often used in
industry and everyday life.

The concentrations of acids and bases are often expressed in terms of pH,
and as an educated person, you should have the skill to convert
concentrations into pH and pOH. The pH is an indication of the hydrogen
ion concentration, [H+].

Strong Acids

Strong acids

Type

Formula

Hydrogenhalides

HCl

HBr

HI

Oxyacidsof halogens,

HClO3HClO4

HBrO3HBrO4

HIO3HIO4

Sulfuric acid

H2SO4

Nitric acid

HNO3

Strong acids are acids that are completely or nearly 100% ionized in
their solutions. Here are some common strong acids:
Ionization of a strong acid HA can be represented by:
HA = H+ + A-xx
where x is the concentration of H+, [H+].
For a strong acid, [H+] = [A-] = concentration of acid
(= x), if x is much greater than 1x10-7 (represented
as e-7). For a very dilute strong acid solution with concentration
less than 1E-7, the pH is dominated by the autoionization of water,
H2O = H+ + OH-, Kw = 1e-14 at 298 K.

The pH and pOH Scales

The pH and pOHscale at 298 K

pH

[H+]

pOH

1

0.1

13

2

0.01

12

3

0.001

11

4

1e-4

10

5

1e-5

9

6

1e-6

8

The pH scale is defined as the negative log of the concentration of H+:
pH = -log[H+]
The pOH scale is defined as the negative log of the concentration of
OH-, [OH-]:
pOH = -log[OH-]
With this scale, calculating the pOH can be done in the same manner
as the pH scale.

Example 1
Calculate the pH of a solution with 1.2345E-4 M HCl

Solution

The solution of a strong acid is completely ionized. Thus,
[H+] = 1.234e-4.

pH = -log(1.234e-4) = 3.909

Discussion
What is the pH for a solution containing 1.234 M [HCl]?
pH = 0.0913

Example 2
Calculate the pH of a stock HCl solution that is 32% by mass HCl.

Solution

The density of such a solution is needed before we can calculate the pH.
Since the density is not on the label, we need to find it from the
Material Safety Data Sheet, which gives the specific gravity of 1.150.
Thus, the amount of acid in 1.0 L is 1150 g.

Discussion
If you require only 1 significant figure, the pH is about 7.

Strong Bases

Strong bases are completely ionized in solution.
For example, KOH dissolve in water in the reaction
KOH = K+ + OH-.

Strong bases

Name

Formula

Sodiumhydroxide

NaOH

Potassiumhydroxide

KOH

Cesiumhydroxide

CsOH

Calciumhydroxide

Ca(OH)2

Relative to strong acids, there are fewer number of strong bases.
Most strong bases are alkali hydroxides. Calcium oxide is considered
a strong base, because it is completely, almost completely, ionized.
However, the solubility of calcium hydroxide is very low.
When Ca(OH)2 dissolve in water, the ionization reaction
is as follows:
Ca(OH)2 = Ca2+ + 2 OH-.
The concentration of OH- is twice the concentration of
Ca2+,
[OH-] = 2 [Ca2+]

Autoionization of Water

The equilibrium product Kw =
[H+] [OH-] is a constant at a definite temperature
due to the autoionization of water,
H2O = H+ + OH-.
At 298 K, Kw = 10-14 and the following
relationship in any aqueous solution is obvious,
pOH + pH = 14 at 298 K.
Of course, when T is higher than 298 K, pH + pOH is slightly less than 14
due to a higher degree of ionization of water. Conversely, at low
temperatures, pH + pOH is larger than 14 due to less degree of ionization.

Confidence Building Questions

What is the pH of a solution containing 0.01 M HNO3?

Answer 2Hint...
You do not need a calculator to evaluate -log (0.01) = 2

What is the pH of a solution containing 0.0220 M Ba(OH)2?
Give 3 significant figures?